1. Field of the Invention
This invention relates to a reference electrode of an insulated gate field effective transistor (hereinafter referred to as "FET") and also to a system for selective detection and measurement of chemical properties of substances which this reference electrode comprises. The term "chemical properties" is used in this invention to include the activity and concentration of ions, the presence and concentration of enzymes, substrates, antibodies, antigens, hormones and reducible gases and the presence, concentration and activity of various chemical substances and biochemical substances. More particularly, the invention relates to a system for selective detection and measurement of the activity and concentration of ions.
2. Description of the Prior Art
Glass electrodes have heretofore been used for selective measurement of quantities of chemical substances in samples. Electrodes of this type are valuable for selective determination of activities of chemical substances, especially various ions in the living body, in the medical and physiological fields. In these conventional electrodes, however, since the resistance of the glass film is about 10 M.OMEGA., a high input resistance amplifier should be used. Further, the glass film is thin and is poor in the mechanical strength. Moreover, when the quantity of a specific chemical substance existing in a very small volume location is to be measured, since the contact area of the glass electrode with the substance should be small, the resistance of the glass film is especially increased. Accordingly, it is difficult to insulate portions other than the top end portion in the electrode. Therefore, when an electrode of this type is employed, it is technically difficult to measure the quantity of a specific chemical substance in a small volume location.
Recently, a novel small size sensor, in which the foregoing defects of glass electrodes are eliminated, was proposed by K. D. Wise et al. in IEEE, Trans. on BME, Vol. BME-21, No. 6, pp. 485-487, 1974. According to this proposal, a gate insulated FET is used, in which silicon nitride, excellent in water resistance, is further coated on the surface of an insulated gate layer composed of silicon dioxide, and the precision of measurement in aqueous solutions is improved by adoption of this structure. Such an FET can be used as a sensor sensitive to the hydrogen ion. In the above report, K. D. Wise et al. suggested that an FET having the above structure would possibly be a sensor capable of selectively detecting various chemical substances. Further, the structure of an FET which can selectively detect and measure various chemical properties is disclosed in U.S. Pat. No. 4,020,830. The disclosed sensor has an insulated gate layer composed of silicon dioxide or silicon nitride and a layer having a chemical selectivity, such as used in a glass electrode, which is coated on the surface of the insulated gate layer. By utilizing this sensor, such ions as sodium ions and potassium ions can be measured.
Still further, Japanese patent application Laid-Open Specification No. 26292/77 discloses an FET sensor comprising an insulated gate layer composed of silicon dioxide, silicon nitride and a top layer selectively sensitive to specific chemical substances, which is coated on the insulated gate layer.
In the above-mentioned FET sensor, the size can be decreased and the defect inherently involved in a glass electrode, namely a high electrode resistance, be eliminated. Therefore, this sensor can be advantageously used as a miniature electrode, especially in the medical and biological fields. When such sensor is used, however, it is indispensable to employ a reference electrode providing a certain potential difference irrespectively of the ion concentration in a liquid to be tested.
Electrodes dipped in an internal reference solution, such as a calomel electrode, a silver chloride electrode and a mercuric oxide electrode, have heretofore been used as reference electrodes, and electrodes of thisin a liquid to be tested.
Electrodes dipped in an internal reference solution, such as a calomel electrode, a silver chloride electrode and a mercuric oxide electrode, have heretofore been used as reference electrodes, and electrodes of this type are also disclosed in U.S. Pat. No. 4,020,830. In such a reference electrode, an internal reference liquid should be contacted with a solution to be tested through a liquid junction. These reference electrodes are divided into three types, namely the pinhole type, the sleeve type and the fiber type according to the structure of the liquid junction. In each type, if the size of the liquid junction is diminished in order to prevent the internal reference solution from flowing out, measurement values become unstable, because of increase of the resistance. Accordingly, it is necessary to maintain a certain size in the liquid junction, and hence, flow-out of the internal reference solution from the liquid junction cannot be avoided. Therefore, if flow-out of the internal reference solution is taken into account, it is not permissible to reduce the quantity of the internal reference solution below a certain level and hence, diminution of the size of the reference electrode is limited. Therefore, even if a very small size FET sensor is used, since the size of a reference electrode to be used in combination is large, a minute substance cannot be measured and the measurement is impossible in a small vessel. For this reason, it has been desired in the art to develop a small size reference electrode capable of allowing an FET to exert characteristics thereof sufficiently.
Recently, several reference electrodes meeting this objective have been proposed. For example, in the 30th Annual Conference on Engineering in Medicine and Biology, J. Janata proposed a reference electrode in which an FET sensor is dipped in an internal reference solution instead of a silver chloride electrode customarily used for the conventional reference electrode and is contacted with a liquid to be tested through a liquid junction. In this proposal, since an FET is used instead of the conventional reference electrode, the size can be decreased to some extent, but since an internal reference solution is employed, decrease of the size is inevitably limited. Further, it is very difficult to manufacture a minute electrode having such structure. Separately, Japanese patent application Laid-Open Specification No. 26292/77 discloses that an FET sensitive to a substance of a constant concentration in a liquid to be tested is used as a reference electrode. As one example of such means, there is proposed a reference electrode comprising silver-silver chloride coated on the surface of a gate region of FET. This electrode acts as a sensor selectively detecting the concentration of chlorine ions. Accordingly, this electrode can be used as a reference electrode in the tissue of a living body where the chloride ion concentration is constant. This electrode, however, is defective in that since the potential is changed according to the chlorine ion concentration, it can be applied to limited uses alone.